1. Field of the Invention
The present invention relates to a sheet processing apparatus.
2. Description of the Related Art
Some image forming devices such as photocopiers, printers, facsimiles, and multifunctional peripheral devices are provided with a sheet processing apparatus that processes sheets discharged from the body of the image forming apparatus. For example, the sheet processing apparatus staples the sheets. Some of such sheet processing apparatuses load the discharged sheets on a process tray and align the sheets before stapling.
Japanese Patent Laid-Open No. 2004-51256 discloses an image forming apparatus whose body is provided with a lateral register correction unit that detects the side edge in a direction perpendicular to the sheet conveying direction (hereinafter referred to as “width direction”) of a sheet and moves the sheet in the width direction so as to correct the position in the width direction of the sheet. The term “lateral register correction” here means position correction in the width direction of a sheet.
By providing such a lateral register correction unit, the lateral register position of a sheet can be aligned with the image forming position. In addition, since the side edge of the sheet can be detected and the sheet can be moved while the sheet is being conveyed, the sheet position can be corrected without reducing the productivity of the image forming apparatus.
In addition, by performing the lateral register correction of the sheet, the sheet can be discharged from the body of the image forming apparatus to the sheet processing apparatus with the position of the side edge in the width direction of the sheet aligned.
However, while the sheet is conveyed from the entrance of the sheet processing apparatus to, for example, a sheet stapling part in the sheet processing apparatus, lateral register displacement, that is to say, displacement in the width direction occurs. Therefore, when sheets are processed, a sheet alignment operation is performed on a process tray on which sheets are temporarily loaded. That is to say, it is necessary to perform a sheet alignment operation on the process tray even after the lateral register correction is performed in the body of the image forming apparatus.
Recently, high productivity has been required not only for image forming apparatus but also for a system including sheet processing apparatus. Therefore, it is necessary to reduce the time for sheet processing operations such as sheet alignment on the process tray.
In addition, when the sheet processing apparatus processes a plurality of copies, during a sheet alignment operation on the process tray, sheet stacks are offset copy by copy. The sheet stacks are thereby loaded on the discharge tray, being offset stack by stack. Thus, the sheet stacks are sorted. However, the larger the offset distance is, the longer time is required for the alignment operation on the process tray. Therefore, in order to achieve high productivity in the entire system, it is necessary to reduce the alignment time concerning sorting.
If there is a malfunction in the unit that aligns sheets, the entire system can go down. This is one of the factors that prevents high productivity from being achieved.